The present invention relates to an ignition system for an internal combustion engine, and particularly to an ignition system for generating a high voltage for ignition twice or more per combustion stroke by repeating conduction/cutoff of a primary current in accordance with an ignition control signal.
In the background art, in a spark igniter for an internal combustion engine using an ignition plug, a so-called multiple ignition method for inducing, not once but repeatedly, a high voltages on the secondary winding of an ignition coil in one combustion stroke to thereby make an ignition plug spark several times for the purpose of improving ignitability of mixture has been proposed as disclosed in JP-A-50-58430.
An example of an igniter using such a multiple ignition method as known in the background art will be described below with reference to FIGS. 8 and 9.
FIG. 8 illustrates a portion corresponding to one cylinder in an independent ignition type igniter provided with independent ignition coils for ignition plugs in cylinders, respectively, of an engine.
In FIG. 8, the reference numeral 1 designates a battery; 2, an engine control unit (hereinafter abbreviated as "ECU"); 3, an ignition coil; 4, an ignition plug; and 5, an igniter unit (ignition circuit).
The igniter unit 5 includes a power transistor (switching device) 9 for controlling conduction and cutoff of a primary current, and a combination of a primary current sensing resistor 10 and a current limiter 11 for serving as a protecting function. The igniter unit 5 is contained in a unit casing integrated with a body casing of the ignition coil.
The ECU 2 calculates the value of a lead angle and the time of current conduction on the basis of the temperature of cooling water, intake pressure, the rotational speed of the engine, etc., and supplies high and low pulses to a base of the power transistor 9 of the igniter unit 5 in proper timing to thereby perform on/off control of the power transistor 9. As a result, the current (primary current) on the primary winding 6 of the ignition coil 3 is conducted and cut off, so that a high voltage is generated on the secondary winding 7 of the ignition coil 3.
The primary winding 6 of the ignition coil 3 is connected, at its one end, to a positive electrode of the battery 1, and connected, at its other end, to a collector terminal of the power transistor 9. The secondary winding 7 is connected, at its one high-voltage-side end, to the ignition plug 4 through a high-voltage diode 8 for preventing premature ignition, and connected, at its other end, to the battery 1.
FIG. 9 shows waveforms in the operation of multiple ignition by conduction and cutoff of the primary current.
In FIG. 9, the reference numeral 12 designates an ignition control signal output repeatedly from the ECU 2 in one combustion stroke; 13, a primary current on the primary winding of the ignition coil 3; and 14, a secondary voltage generated on the secondary winding of the ignition coil 3 and between electrodes of the ignition plug 4.
When the ignition control signal (pulse) 12 is applied to the switching device 9 in the ignition circuit, the primary current 13 begins to flow after a time lag proportional to a time constant defined by inductance and resistance. Under such a condition, the level of the ignition control signal becomes low in proper ignition timing after the start of current conduction. As a result, the primary current is cut off, so that a high voltage is generated between electrodes of the ignition plug 4. The ignition control signal is applied again to the switching device 9 with the passage of time determined by the ECU 2, so that the primary current begins to flow again. In this occasion, however, the next current conduction in the primary winding restarts in a condition that the previous discharging operation has not completed yet. Hence, the primary current jumps up by several amperes because of the influence of residual energy. Then, the primary current begins to flow with a time lag proportional to the time constant. In the third-time current conduction, the primary current jumps up more greatly in terms of waveform.
It can be conceived that the current sensing resistor 10 and the current limiter 11 are used to prevent the primary current from exceeding a predetermined limit value to thereby protect circuit elements even if the aforementioned jump-up occurs. Such a current limiting function keeps the primary current constant by making the power transistor unsaturated. Accordingly, the energy consumed in this condition depends on the power transistor, so that the power transistor generates excessive heat. Moreover, in repeated multiple discharge, it is wasteful to keep the conducting current within the limit value because it is ideal to perform repeated conduction and cutoff of the primary current in a time as short as possible. Incidentally, current limiters for limiting such a primary current by methods other than the multiple ignition method are disclosed in JP-A-5-180134, JP-A-9-291870, JP-A-9-324731, etc.
In the case of the multiple ignition method, the following technique has been proposed for the purpose of supplying stable ignition energy.
For example, JP-A-57-28871 has proposed a system having a device for detecting a primary current flowing in the primary winding, and a device for detecting a secondary current flowing in the secondary winding. The proposed system further has a multiple ignition signal generating circuit to execute a cycle in which a power switching device for conduction and cutoff of the primary current is turned on again to conduct the primary current when the current in the secondary winding is reduced to a value not larger than a predetermined value (that is, when an igniting operation is performed), and the power switching device is turned off when the primary current reaches the predetermined value. In this manner, the proposed system performs multiple ignition.
On the other hand, JP-A-3-121273 has disclosed means of integrating the primary current and controlling the cutoff of the primary current when the integrated primary current value exceeds a set value.